This invention relates to the formation of a fine pattern by a lithography process in the manufacturing of a semiconductor integrated circuit.
A recent tendency of semiconductor devices is to make the design rule as fine as possible, and semiconductor chips of the order of 0.25 .mu.m have been available in market. According to this tendency, in the lithography the exposure wave length, being decreased, has been changed from g-line (436 nm) through i-line (365 nm) to KrF excimer laser (248 nm). A stepper using an ArF excimer laser (193 nm) has been developed for the next generation lithography. However, the development is retarded because the lens material absorbs the ultra short wave light beam such as ArF excimer laser beam. In the circumstances, in the lithographic technique using the KrF excimer laser, research is conducted on ultra resolution technique.
In general, the limit resolution of optical lithography according to a reduction projection exposure method is proportional to the exposure wave length, and inversely proportional to the numerical aperture of the projection lens. The formation of a pattern of the order of 0.3 .mu.m has been achieved by using a KrF excimer laser (a wave length of 148 nm) and a projection lens whose numerical aperture is 0.4 to 0.5.
One of the ultra resolution techniques which improve the resolution limit in the reduction projection exposure method, is a method using an alternating phase-shifting mask (Levenson phase-shifting mask).
An example of the formation of a pattern which uses a conventional alternating phase-shifting mask, will be described.
FIGS. 8A through 8C are sectional views showing a pattern forming method using a conventional alternating phase-shifting mask. In those figures, reference numeral 21 designates a positive resist; 22, a substrate; 23A and 23B, exposure light beams; 24, a phase-shifting mask; 25, opaque regions; and 26A and 26B, transparent regions (transparent regions). The transparent regions 26B are shifted 180.degree. in the phase of the exposure light beams 23 with respect to the transparent regions 26A. The exposure light beam 23A irradiates the mask 24, and the exposure light beam 23B passed through the transparent regions of the mask are imaged onto the resist 21.
As shown in FIG. 8A, first the substrate 22 is coated with the positive resist 21. The latter 21 is a chemical amplification type resist for a KrF excimer laser, and it is applied to the substrate 22 to a thickness of 0.5 micron. Next, the positive resist is subjected to exposure through the phase-shifting mask 24.
The exposure conditions of the exposure device (or stepper) are an exposure wave length .lambda.=248 nm, a numerical aperture NA=0.48, coherent factor a .sigma.=0.30. The alternating phase-shifting mask is of quartz. As shown in FIG. 8B, the quartz are ground in correspondence to the transparent regions 26B. The light beams passing through the transparent regions 26B are inverted 180.degree. in phase with respect to the light beams passing through the transparent regions 26A.
First, the exposure light beam 23A irradiates the mask 24, and diffracted according to the pattern density. In the case of the alternating phase-shifting mask, the transparent regions on both sides of each opaque region are different 180.degree. in phase from each other, and therefore with a period pattern the zero-th order and even-number-th order light beams are canceled out. On the other hand, the odd-number-th order light beams such as .+-. first, third, and fifth orders light beams are generally diffracted at an angle which is half of that in usual mask. In general, the angle of a light beam passed through a projection lens is finite, and therefore the pattern resolution is a pattern period which can pass through the mask. In the alternating phase-shifting mask, in general the light beam is diffracted at an angle which is half of that of the usual mask, and therefore ideally a high resolution two times the usual mask is obtained.
In order to obtain a high resolution with an alternating phase-shifting mask, it is necessary to arrange spatial optical phases (to increase the coherency). The unit indicating the degree of coherency is the ratio (coherent factor) .sigma. of the NA of the projection lens to the NA of the irradiation system. As the ratio a is decreased, the optical coherency is increased. In general, in the case where the mask is irradiated with the stepper, the coherent factor (.sigma.) is of the order of 0.5 to 0.8; however, in the case where the alternating phase-shifting mask is employed, the coherent factor is of the order of 0.2 to 0.4.
After the pattern exposure as shown in FIG. 8B, a PEB (post exposure baking) processing is carried out, and a developing processing is performed with an ordinary alkaline solution for sixty (60) seconds, to form a resist pattern (FIG. 8C). According to this pattern forming method, a line and space pattern of 0.16 .mu.m is obtained which is much smaller than the exposure wave length 248 nm (0.248 .mu.m). In general, the alternating phase-shifting mask is effective in obtaining a fine periodical pattern. Hence, research has been conducted on the application of it to a DRAM device.
However, the alternating phase-shifting mask suffers from the following problems: Even if patterns are equal in line width, adjacent patterns are different in distance from one another, then the resist dimension projected onto the wafer is changed. Hence, although the dimensional control of the line width of the gate pattern of a logic device is considerably important, the logic device includes a number of random patterns, and therefore the application of the alternating phase-shifting mask has not been very studied.
In FIG. 8C, a pattern 21X and a pattern 21Y are resist patterns which are obtained by transferring light through a mask pattern which is so designed that the former 21X is a 0.16 .mu.m line and space pattern and the latter 21Y is a 0.16 .mu.m line/0.48 .mu.m space pattern. In this case, the actual dimensions of the resist pattern transferred onto the wafer are as follows: The pattern 21X is 0.16 .mu.m, while the pattern 21Y is 0.20 .mu.m, and therefore, the difference between those two patterns is 0.04 .mu.m. In the formation of an ordinary transistor gate, in general its tolerance is .+-.10% of a line width. Hence, in the case of a line width of 0.16 .mu.m, the tolerance must be within 0.03 .mu.m. Accordingly, the pattern forming method using the conventional phase-shifting mask is not applicable to the pattern formation of the transistor gate which must be considerably high in dimensional accuracy.